MIAMI (Reuters) - A record number of manatees died in Florida waters this year and the leading killer of the endangered mammals was a toxic algae bloom in the Gulf of Mexico, according to the state's Fish and Wildlife Conservation Commission.

The commission's research institute said it documented 803 manatee deaths in state waters between January 1 and December 13, the most for any year since record-keeping began in 1974.

That was more than double the 392 manatee deaths in Florida in 2012 and up sharply from the 453 deaths recorded in 2011.

"The previous record was 766 manatee deaths and that was in 2010," commission spokesman Kevin Baxter said on Friday. "That was a year when cold weather was a major factor," he added.

Baxter said a Red Tide that killed 276 manatees in the Gulf, in an area centered largely off Ft. Myers, was the single leading cause of death in 2013.

Naturally occurring algae sometimes grow out of control for unknown reasons, turning the water red and producing a suite of neurotoxins deadly to fish, sea turtles, birds and marine mammals.

The algae settles on the sea grasses that manatees eat, affecting their nervous system and ultimately causing them to drown, Baxter said.

"The previous record for Red Tide deaths that we've documented was 151 and that was in 1996," he said.

Algae could also be a factor behind at least 92 manatee deaths this year in the Indian River Lagoon, in central Florida along the state's Atlantic coast, Baxter said.

He said the exact cause of the deaths was still undetermined. But an algal bloom had clouded the water in parts of the lagoon, blocking out sunlight, inhibiting sea grass growth, and possibly forcing the manatees to eat something that is killing them off.

"We do believe that it may have something to do with a dietary change, Baxter said.

An aerial survey conducted in 2011 showed that there was a "minimum count" of at least 4,834 manatees living in Florida's waters, Baxter said.

The West Indian manatee, the whiskered and wrinkled "sea cow" known to Floridians, is related the African and Amazon species and to the dugong of Australia. It grows to at least 10 feet and more than 1,000 pounds (450 kg).